1. Field of the Invention
The present invention relates to a compact disc apparatus (CD), CD-ROM, mini disc apparatus (MD: registered trademark of SONY), or other optical disc apparatus.
Further, the present invention relates to a tracking error signal calculation circuit used in the optical disc apparatus.
2. Description of the Related Art
An optical pick-up is used for recording data or reading recorded data along a track (guide groove) of a disc recording medium of an optical disc apparatus. In the optical pick-up, optical parts such as a semiconductor laser, photodiode (PD), prism, and object lens are mounted.
At the time of recording or reading data to or from the disc recording medium, in order to eliminate the influence of the face vibration and wobbling of the track of the disc recording medium, and the face vibration and wobbling by the tilting of the turntable of the disc drive and precession motion, and the like, focusing servo control and track servo control are carried out.
In focusing servo control, the object lens for focusing (exactly focusing) the laser beam light emitted from a semiconductor laser on the recording surface of the disc recording medium is positioned with respect to the surface of the disc recording medium.
In tracking servo control, the optical pick-up is positioned in the radial direction of the disc recording medium to position the laser beam emitted from the laser at a desired track of the disc recording medium (to place it on-track).
A focus error signal is used for the focusing servo control, while a tracking error signal is used for the tracking servo control.
Usually, signals detected by two photodiodes are computed by the push-pull system to calculate the tracking error signal.
An offset will appear in the tracking error signal due to the push-pull system. When there is an offset, even if the tracking error signal indicates 0, if the tracking servo control is carried out by using that tracking error signal, the beam of light of the semiconductor laser will deviate from the center of the track, so it suffers from a disadvantage of poor control when performing the tracking control.
As factors behind the appearance of the offset in the tracking error signal, there are deviation of the optical axis of the object lens, tilting of the disc recording medium in the radial direction, an imbalance in groove shape on the disc recording medium, etc.
As the method for reducing the offset due to the above factors, a variety of countermeasures have been made. For example, refer to "Optical Disc Technology", supervised by Morio Onoe, Radio Gijutsusha, page 91 to page 98.
The inventors of the present application found that just the above-mentioned countermeasures for reducing the offset were not sufficient and that offset still existed in the tracking error signal. Accordingly, when using a tracking error signal calculated by the conventional push-pull system, the problem is encountered that accurate and stable tracking servo control cannot be carried out in the optical disc apparatus.